Prilling tower

ABSTRACT

A PRILLING TOWER INCLUDES A SHAFT THE WALLS OF WHICH ARE MADE OF A FLEXIBLE NONWETTABLE CORROSION RESISTANT MATERIAL HAVING LOW THERMAL CONDUCTIVITY. THE WALLS ARE PREFERABLY FORMED OF PLASTIC MATERIAL OR PLASTIC COATED FABRIC. THE WALLS MAY BE EMPLOYED FOR COVERING THE SHAFT WALLS OF A CONVENTIONAL PRILLING TOWER, OR THE WALLS MAY BE ATTACHED TO A RIGID FRAMEWORK. IN ONE FORM OF THE INVENTION, THE TOWER COMPRISES AT LEAST TWO SHAFTS HAVING ONE SHAFT WALL IN COMMON.

7 Oct 26, 1971 "LIAYRS-OLOV DAHLBYOM 3,61

PRILLING TOWER Filed Jan. 29, 1969 United States Patent "ice 3,615,142PRILLING TOWER Lars-Olov Dahlbom, Garpenberg, Sweden, assignor to KemaNord AB, Stockholm, Sweden Filed Jan. 29, 1969, Ser. No. 794,976 Claimspriority, application Sweden, Jan. 30, 1968, 1,234 68 Int. Cl. B29c23/00; 322d 23/08 U.S. Cl. 18-2.7 11 Claims ABSTRACT OF THE DISCLOSUREBACKGROUND OF THE INVENTION The prilling process is a known manner ofproducing granular products from liquid melts and concentratedsolutions. The liquid is fed through suitable nozzles and divided intodroplets which fall through a gaseous medium, usually air, which coolsthe droplets so that they solidify into spherical granules or pellets.When the liquid fed through the nozzles is a solution, solidification isaccompanied by partial or complete evaporation of the solvent used.

The prilling technique is industrially employed among other things inthe manufacture of urea prills and ammonium nitrate prills as well asother substances which form melts or concentrated solutions,particularly those having water as a solvent.

A prilling plant generally includes a high tower which encloses aprilling shaft. The nozzles are usually mounted at the top of the shaftand the droplets fall downwardly in the shaft and contact a steram ofupwardly moving cooling air introduced at or near the bottom of theshaft. In order to give the droplets sufficient time to solidify intoprills during their fall, shafts of considerable height are required.For example, fertilizer grade ammonium nitrate prills are manufacturedin towers which often are more than 100 meters in height. Early prillingtowers were of brick construction, and later types of towers haveusually been built as monolithic constructions of reinforced concrete.Towers of the height and construction mentioned above are very heavy andbulky and require a high initial investment.

Especially when the nozzles are not operating properly, the walls of theprilling shaft become covered with solid deposits of the prilledmaterial. Such deposits disturb the flow pattern within the shaft, andaccordingly, as the deposits build up on the walls of the shaft, itbecomes necessary to interrupt operation of the apparatus to clean andwash the shaft with water. The salt solutions thus formed have acorrosive action on the walls of the shaft.

In order to reduce the build-up of deposits on the shaft walls and toeliminate corrosion, it is common practice to cover the shaft walls withmetals such as stainless steel or aluminum. This type of construction isoften used where concrete walls are provided, and these materials havealso been employed to cover the shaft walls wherein the shaft is of aconstruction other than concrete.

3,615,142 Patented Oct. 26, 1971 SUMMARY OF THE INVENTION In the presentinvention, the above-discussed disadvantages of conventional prillingtowers are eliminated by providing a prilling tower wherein the walls ofthe shaft are made of flexible, nonwcttable corrosion resistant materialhaving low thermal conductivity. Suitable materials are sheets ofplastic or plastic coated fabric. Plastics which are particularly suitedfor use in the present invention are polyethylene, chlorosulfonatedpolyethylene and polyvinyl chloride. A suitable fabric for use in theinvention is nylon.

The shaft walls of flexible material as discussed hereinabove accordingto the present invention may be employed for covering the shaft walls ofa conventional prilling tower formed of the usual materials.Additionally, the material of the present invention may be employed toform intermediate partition walls dividing conventional prilling towersinto a plurality of shafts.

A preferred embodiment of the invention comprises a prilling towerincluding a rigid framework formed of metal or the like, and wherein theplastic Wall material is attached to the framework to form the shaft.

Use of the material according to the present invention in the shaftwalls of prilling towers provides a number of substantial advantagesover prior art constructions. The flexible wall material according tothe invention is of lightweight construction and accordingly it can beadded to existing prilling towers without substantially increasing thetotal weight of the tower. Additionally, the material enables newlightweight low cost prilling towers to be readily constructed.

The flexible material is easy to mount in operative position and maywithout the use of any special tools or equipment be easily shaped toprovide shafts having a desired horizontal cross-section. Furthermore,the horizontal crosssection may be readily altered as by making theshaft walls movable or by providing the tower with intermediatepartition walls of similar flexible material.

The wall material of the present invention require no maintenance and itis not corroded by the melt or the solution to be prilled or by diluteaqueous solutions containing the same or other chemicals.

A prilling tower comprising a rigid framework having flexible shaftwalls attached thereto and providing a shaft of generally rectangularcross-sectional configuration has the additional advantage that theproduction capacity of the plant can be increased in a simple andeconomical manner by adding one or more adjoining towers of similarconstruction in such a manner that adjacent towers define shafts havingat least one shaft wall in common.

A prilling tower according to the present invention offers significantoperating advantages since the low thermal conductivity of the flexiblematerial of the shaft wall and its lower surface tension towards themelt or solution to be prilled causes the deposits on the shaft walls toadhere poorly to the wall so that the deposits can be easily removed.

Additionally, the shaft walls are attached to the framework in such amanner that they may be flexed. This enables the deposits on the shaftwalls to be removed therefrom by shaking or vibrating the Walls.Accordingly, interruptions for cleaning with water and the accompanyingformation of crystal pulps or solutions in the bottom of the tower areeliminated.

The operational advantages of providing poor adherence of deposits andenabling the deposits to be removed by shaking or vibrating the shaftwalls are particularly important when the droplets are formed by meansof high pressure nozzles or by means of centrifugal sprayers such asdiscs or perforated cylinders which rotate with relatively high speed.Such devices are often used in the manufacture of products such asfertilizers or the like.

The prilling process can be carried out with an optimal flow patternwithin the shaft since the horizontal crosssection of the shaft can bemade smaller than usual withoutrisk of disturbances and interruptions ofoperation.

Since the cross-section of the shaft can be readily changed as discussedhereinabove, the flow pattern in the shaft can be varied therebyenabling a single prilling tower to be utilized for the manufacture ofprills having different properties such as size, porosity and chemicalcomposition, etc.

When high and uniform quality is required as to prill size and prillsize distribution, shape, moisture content, porosity and mechanicalstrength, it is necessary to permanently maintain within narrow limitsall of the optimum operating conditions. In such a case, the sprayingdevice may suitably comprise a multi-hole-orifice nozzle which isconstructed in such a known manner as to enable interference freeoperation with droplet formation occurring at a point spaced from theorifices.

This is achieved by regulating the pressure on the melts or solutionbeing forced through the nozzles so that the melt or solution leaves theorifices as an unbroken stream having a substantially laminar flow. Thestreams under the influence of surface tension at a suitable level inthe shaft are disrupted into droplets. The cooling air is regulated tosolidify the droplets to prills and to control porosity at such lowerlevel of the shaft where the droplets have attained a spherical shape. Aprilling tower having a small horizontal cross-section and which enablesthe flow pattern of the shaft to be varied makes it possible toregulate, control and maintain ideal conditions for such a sequence ofprocess steps.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a vertical sectionthrough a prilling tower according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing,the prilling tower includes an elongated rigid hollow framework formedof suitable metallic members interconnected with one another to providean open framework of substantially rectangular cross-sectionalconfiguration. The walls 12 of the tower are formed of a flexiblenonwettable corrosion resistant material having low thermal conductivityas dis cussed hereinbefore and preferably comprise a sheet of nylonfabric coated with polyvinyl chloride. The flexible walls are attachedto the framework by any suitable means. They may for example be attachedto the upper and lower edges of the framework and by means of a numberof firmly braced corrosion resistant wires 14 positioned inside theflexible walls be pressed against the corners and sides of theframework. The fabric may be further fastened to the framework at spacedpoints along the side thereof in such a manner that the fabric maintainsits flexibility.

A pair of sloping side walls 1 8 and are provided at the bottom of thetower and are of a louver construction to enable air to pass freelytherethrough. A conveyor belt 24 is disposed beneath the sloping walls18 and 20 for receiving prills from the tower.

A conduit 30 connected with a suitable source of liquid melt or solutionis provided with a plurality of nozzles 32 for producing the desireddroplets. These nozzles are disposed within an enclosure 40. A pluralityof fans 42 are mounted in the enclosure for drawing air upwardly throughthe shaft defined by the framework and flexible walls and thenceoutwardly of the enclosure 40.

The tower may also include a partition indicated by phantom line -Pformed of the same material as the walls of the tower for dividing thetower into a plurality of adjacent shafts.

As this invention may be embodied in several forms without departingfrom the spirit or essential characteristics thereof, the presentinvention is therefore illustrative and not restrictive, and since thescope of the invention is defined by the appended claims, all changesthat fall within the metes and bounds of the claims or that form theirfunctional as well as conjointly cooperative equivalents are thereforeintended to be embraced by those claims.

What is claimed is:

1. A prilling tower which comprises:

(a) a rigid vertical framework;

(b) walls attached to said framework, said walls comprising flexiblenon-wettable corrosion resistant material having low thermalconductivity;

(c) said walls forming a vertical shaft of generally rectangularcross-sectional configuration in said prilling tower;

(d) a spraying means located at the upper portion of the prilling towerto divide a liquid flowing therethrough into droplets;

(e) said spraying means being positioned so that said droplets will falldownwardly in said shaft;

(f) a gas inlet located at or near the bottom of said tower andpositioned so that said gas will flow into said shaft and contact saidfalling droplets; and

(g) a product outlet located at the lower level of said shaft.

2. A prilling tower as defined in claim 1 wherein said walls are made ofsheets of plastic material.

3. A prilling tower as defined in claim 1 wherein said walls compriseplastic coated fabric.

4. A prilling tower as defined in claim 1 wherein said walls comprisenylon fabric coated with polyvinyl chloride.

5. A prilling tower as defined in claim 1 wherein said walls are made ofplastic selected from the group consisting of polyethylene,chlorosulfonated polyethylene and polyvinyl chloride.

6. A prilling tower as defined in claim 1 wherein the prilling towerincludes at least two shafts having one shaft wall in common.

7. The tower of claim 1 wherein said walls are movable.

"8. The tower of claim 1 wherein the bottom of the tower comprises apair of sloping side walls which are of a louver construction to providesaid gas inlet.

*9. The tower of claim 1 wherein said spraying means are disposed withinan enclosure.

10. The tower of claim 1 wherein a plurality of fans are mounted in saidenclosure in order to draw air upwardly through said shaft and outwardlythrough outlets of said enclosure.

11. The tower of claim 1 wherein said walls are attached to saidframework in such a manner that they may be flexed.

References Cited UNITED STATES PATENTS 2,403,476 7/ 1946 Berry et al.

2,574,357 11/1951 Stammer et al. 264-13 X 2,876,927 3/ 1959 Henning.

2,921,335 1/ 1960 Bowers et al. l'82.7

2,947,115 8/1960 Wood 182.4 X

3,279,889 10/1966 Rychly 23252 2,858,903 11/1958 Goelz et al.

2,898,625 8/1959 'Chao 182.7

FOREIGN PATENTS 1,330,546 5/1963 France.

J. SPENCER OVERHOLSER, Primary Examiner R. L. SPIOER, 111., AssistantExaminer US. Cl. X.R. 264--13

